1. Field of the Invention
This invention relates to the fields of anti-block agents and processing release agents for vulcanized vinyl acetate-ethylene elastomeric copolymers.
2. Description of the Prior Art
Vinyl acetate-ethylene (VAE) elastomeric copolymers are extremely self-sticking or cohesive in nature and, in fact, can be processed into excellent adhesives. Compounds of VAE also exhibit this extreme self-stickiness and, unless parting sheets such as polyethylene film are used to separate stacked pieces of the uncrosslinked compositions, the pieces will weld together to such a degree that they cannot be separated. Even after crosslinking, i.e., vulcanizing, molded articles of VAE will strongly cohere such that, for convenience, parting sheets must be used.
Several anti-block or slip agents are known for polyethylene, such as fatty acid amides, e.g., oleamide, stearamide, erucamide and behenamide. Ridgeway et al. (U.S. Pat. No. 3,474,058) describe the use of a metallic salt of a fatty acid in combination with a fatty acid amide for improved slip and anti-block properties in ethylene-vinyl acetate (EVA) copolymer films of low vinyl acetate (VA) content. In contrast, the high VA content VAE copolymers are elastomeric and will not form useful films. Fatty acid amides admixed with compounds of VAE impart no or limited anti-blocking capability. The addition of metal salts of fatty acids, with or without admixed fatty acid amides, has no effect on the anti-block characteristics of VAE compounds.
In addition to the pronounced self-sticking nature of VAE elastomeric copolymers, these resins when hot are extremely adhesive, especially to metallic surfaces. Consequently, VAE elastomeric copolymers are difficult to process due to the tendency of the hot elastomers to adhere to the surfaces of equipment with which they are in contact. For example, when compounded VAE resins are being blended in production scale apparatus such as a Banbury mixer, a considerable portion of a completed batch will not readily be dislodged from the machine but will adhere to the mixing rotors and mixing chamber surfaces where it is removed only with some difficulty and at a significant loss of production efficiency. As the size of the mixing equipment or other process apparatus is reduced, the difficulty of processing VAE elastomeric copolymers increases. In a laboratory scale Banbury mixer, all of the VAE resin will be retained in the mixing chamber and can only be removed by hand in a time consuming operation.
The addition of such compounding ingredients as fillers, plasticizers, antioxidants and curing agents has little or no effect on the adhesion of VAE resins to the surfaces of mixing and processing equipment other than to alter green strength (green strength being increased by reinforcing fillers and decreased by most other additives). Some improvements in release characteristics of VAE resins are observed with increasing green strength.
The lubrication or the plasticization of polymers which are chemically and physically dissimilar from VAE elastomeric copolymers are known. Ridgeway et al., supra, obtain improved slip in an EVA film by incorporating therein a combination of a metallic salt of a fatty acid and a fatty acid amide. EVA films are more nearly related to polyethylene films in their properties and behave in a manner which is altogether different from that of the VAE resins which as previously noted, cannot be formed into films.
Stapfer (U.S. Pat. No. 3,578,621) teaches the use of diesters as internal-external lubricants in structural resins such as polyvinyl chloride (PVC) and acrylonitrile-butadiene styrene (ABS) copolymer. Hermann et al. (U.S. Pat. No. 3,541,041) teach the use of hydroxy alkylated fatty amines as mold release agents for polyamides. Huber et al. (U.S. Pat. No. 3,331,802) describe the preparation of a plasticizer for PVC by an esterification of a mixture of mono and dibasic acids with a trihydric alcohol and a glycol component to give a polymeric ester composition of molecular weight 700 to 3000. Boussely (U.S. Pat. Nos. 3,691,219 and 3,864,295) describes the preparation of a synthetic glyceride which has lubricating properties and prevents sticking of, e.g., PVC compositions to processing equipment or molds at 210.degree. C. (410.degree. F.). Recent investigation has shown that glycerides are ineffective as release agents for VAE resins when used alone. McAninch et al. (U.S. Pat. No. 3,268,463) teach the use of a fatty acid ester of a polyhydric alcohol as a release agent in an asphaltic blend containing 1 to 30% EVA copolymer (from 27 to 34% VA content). After the hot liquid asphaltic blend has been applied to a metallic surface and allowed to harden on cooling, the release agent becomes effective. Commonly assigned copending patent application Ser. No. 783,527 of Meeks discloses and claims a VAE elastomeric copolymer of improved processing release characteristics imparted thereto by a mixture of hydrogenated glyceride and stearic acid. However, vulcanized VAE resins containing this mixture, while demonstrating good processing release characteristics, in some compounds and under certain storage conditions exhibit "bloom", a visible exudate on the surface of the product. It is also well known in the art that release for some polymers can be obtained from lubricants such as paraffin wax and stearic acid. However, neither is effective when used alone in VAE copolymers.